1. Technical Field
The present invention relates to a non-contact tracing control apparatus which fetches positional data in succession while tracing the shape of a model by using detectors, to originate NC data or effect tracing work, and more particularly, to a non-contact tracing control apparatus in which a system for driving optical distance detectors is improved.
2. Background Art
A conventional tracing control apparatus uses a contact-type probe as a tracer head, and obtains displacement information by bringing the probe into contact with the surface of model thereby to control the feeding speed or to originate NC data.
If the surface of a soft model is traced by using this tracer head, however, the model surface will be damaged by the pressure with which the contact-type probe is held against the model, and thus there has recently been developed a non-contact tracing control apparatus in which a distance detector for detecting the distance to the model surface in a non-contact manner is fixed to the distal end of the tracer head. An optical distance detector is used for this distance detector.
FIG. 4 illustrates the principle of the optical distance detector, where a semiconductor laser oscillator 43 is excited by a laser driver circuit 42, and outputs a laser beam 44; the laser beam 44 is condensed by a projector lens 45 and applied to the surface of a model 20c; the applied laser beam 44 is reflected by the surface of the model 20c, and part of a reflected light beam 46 is converged on a position sensor 48 by a light receiving lens 47.
The position sensor 48 is used for converting the reflected light beam 46 into an electrical signal corresponding to the position of the converging point and the quantity of light. Therefore, when the model 20c is on a point P0 as illustrated, the reflected light beam 46 is converged on the center of the position sensor 48, and when the model 20c is on a point P1 or P2, the reflected light beam 46 is converged on the upper left or lower right portion of the position sensor 48. Since the position sensor 48 outputs a detection signal D corresponding to the position and quantity of the thus-converged reflected light beam 46 between the optical distance detector and the model 20c, a signal corresponding to the distance can be obtained by amplifying the detection signal D in a predetermined converter circuit.
This optical distance detector can detect only one-dimensional displacement information, and therefore, in a conventional tracing control apparatus, in order to obtain data in a direction normal to the surface of a model, which data are required to calculate a tool offset, for example, two or three optical distance detectors are employed to simultaneously determine the coordinates of three different points on the model surface, and a vetor in the normal direction is calculated from the coordinates of the three points. According to another attempt, a zigzag tracing path is generated, measurements are successively made on this tracing path by an optical distance detector to obtain the coordinates of three points, and a vector in the normal direction is calculated from the thus-obtained three successive coordinates.
With the first attempt mentioned above, when detecting the coordinates on the model surface by the two or three optical distance detectors fixed on a tracer head, however, a reflected light beam from the model surface is made incident on a position sensor or sensors of the other optical distance detector or detectors, as measurement points on the model surface are brought closer to one another, thus causing interference therebetween and making an accurate measurement of the distance impossible.
If the measurement points are made relatively more distant from one another, the interference can be prevented, but the gradient of the model surface will be unable to be accurately measured during a tracing operation.